Kepler, the paparazzi of the celestial stars, takes snapshots of oscillations that can can be used to tell the size and age of the star. As a star "burns" hydrogen in fusion reactions, helium builds up in the star's core. Helium is more dense than hydrogen, and since waves travel more quickly through denser material, waves travel faster through the core as helium builds up there. Waves that go straight through the center (white) line and waves that bounce around outside the core (colored lines) produce oscillations in surface brightness. Travis Metcalfe, National Center for Atmospheric Research